We estimate the thermal pressure \(p_{\mathrm {th}}=nT\)  in the PDR surfaces from the gas density estimated above and the temperature predicted by the respective PDR model at the estimated \(G_0\) . Fig. REF  shows the thermal pressure versus incident radiation field in a number of regions from our study and from the literature. Our data show a clear trend of increasing thermal pressure with increasing strength of the radiation field. A similar trend is apparent in studies of fine-structure line emission from PDRs [1]} as well as in studies of the CO ladder in PDRs [2]}, albeit that those relations are displaced somewhat. For the study of [1]}, this displacement reflects an overestimate of the incident FUV field in several of the sources. Specifically, they adopted a \(G_0\)  of 5000 for NGC 1977S (the bright region in the southwest of NGC 1977), while we derive a value of 1100. After correction, the data for this particular source agrees, not surprisingly as both analysis are based upon fine-structure line studies. Likewise, the \(G_0\)  value adopted for the Orion Bar by [1]} is somewhat higher than current estimates [5]}, [2]}. The results of the studies by [2]} and [8]} are displaced toward higher pressures. Both these studies rely on high-J CO transitions with high critical densities. Consequently, clumps or gas density enhancements dominate the emission in these lines, whereas the fine-structure lines measure the interclump gas. The offset of these studies might be due to the importance of self-gravity in the clumps, which will dominate the pressure distribution. Hence, the high-J CO lines may not be good measures of the pressure in the [C ii]-emitting PDR layers. As a corollary, we do not expect that constant-pressure PDR models will be applicable to these environments. [2]} note that specifically the [C ii] emission in their sources (bright PDRs associated with massive star formation) predominantly comes from lower-density interclump gas, that fills the beam.
